1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Wireless communication systems typically include a plurality of base stations or access points that provide wireless connectivity to mobile units within a geographical area. The device that provides the wireless connectivity and the geographic area are both conventionally referred to as a cell. The air interface between the base station or access point and the mobile unit supports one or more downlink (or forward link) channels from the base station to the mobile unit and one or more uplink (or reverse link) channels from the mobile units to the base station. The uplink and/or downlink channels include traffic channels, signaling channels, broadcast channels, paging channels, pilot channels, and the like. The channels can be defined according to various protocols including time division multiple access (TDMA), frequency division multiple access (FDMA), code division multiple access (CDMA), orthogonal frequency division multiple access (OFDMA), as well as combinations of these techniques. The geographical extent of each cell may be time variable and may be determined by the transmission powers used by the base stations, access point, and/or mobile units, as well as by environmental conditions, physical obstructions, and the like.
Conventional hierarchical wireless communications include a central element such as a Radio Network Controller (RNC) or a Base Station Controller (BSC). The central controller coordinates operation of the base stations. For example, when data is available for a target mobile unit, the RNC may transmit paging messages to the target mobile unit via one or more base stations or node-Bs. The target mobile unit may establish a wireless link to one or more of the base stations in response to receiving the page from the wireless communication system. A radio resource management function within the RNC receives the voice and/or data and coordinates the radio and time resources used by the set of base stations to transmit the information to the target mobile unit. The radio resource management function can perform fine grain control to allocate and release resources for broadcast transmission over a set of base stations.
One alternative to the conventional hierarchical network architecture is a distributed architecture including a network of access points, such as base station routers or eNodeBs (eNBs), which implement distributed communication network functionality. For example, each base station router or eNB may combine RNC and/or packet data serving node (PDSN) functions in a single entity that manages radio links between one or more mobile units and an outside network, such as the Internet. Base station routers and eNBs wholly encapsulate the cellular access technology and can serve as a proxy for functionality that utilizes core network element support to provide equivalent IP functions. For example, IP anchoring in a UMTS base station router may be offered through a Mobile IP Home Agent (HA) and the GGSN anchoring functions that the base station router proxies by equivalent Mobile IP signaling. Compared to hierarchical networks that use centralized control, distributed architectures have the potential to reduce the cost and/or complexity of deploying the network, as well as the cost and/or complexity of adding additional wireless access points, e.g. base station routers and/or eNBs, to expand the coverage of an existing network. Distributed networks may also reduce (relative to hierarchical networks) the delays experienced by users because packet queuing delays at the separate RNC and PDSN entities in hierarchical networks may be reduced or removed.
Base stations, base station routers, eNBs and other access points are typically assigned cell identifiers that can be transmitted to the user equipment operating in the areas served by these devices. User equipment can use the identifiers for operations including hand off between different cells. For example, in Long Term Evolution (LTE) wireless communication systems, a physical cell identifier (PCI) is allocated to each cell in the distributed communication system. The cell identifiers can also be used to populate neighbor lists. For example, in LTE, lists of neighboring cells are populated automatically using automatic neighbor relationship (ANR). The PCI values have to be assigned to the cells before the automatic neighbor relationships can be established.